Sheikh what happens if a person uttered a kufr or shirk word and says shahadah sincerely but does not do tawbah and dies on that state

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Whoever proclaims the Shahaadah (testimony of faith) sincerely and truthfully, his Shahaadah entails his repentance of all past sins, including the word (of Kufr [disbelief] or Shirk [polytheism]) that he had uttered. In fact, saying the Shahaadah with sincerity essentially entails repentance of all past sins, as narrated in the famous Hadeeth about the man on whose balance of deeds a card bearing ‘La Ilaaha Illa Allah’ (none has the right to be worshipped but Allah) will be placed on the Day of Judgment. Commenting on this Hadeeth, Shaykhul-Islam Ibn Taymiyyah said in Minhaaj As-Sunnah: “This will be the situation of whoever says the Shahaadah sincerely and truthfully like this man did.” [End of Quote]

Hence, it is hoped that this person would be granted salvation on the Day of Judgment. As for the one who commits an act of Kufr by, for instance, denying any of the indisputably established facts of Islam, his faith is not restored unless he says the Shahaadah and affirms what he has denied. If he does not do that and dies while denying any of the indisputably established facts of Islam, then he is not a Muslim.

Allah Knows best.